This invention relates generally to low-friction mechanical transmissions for converting between rotary and linear motion, and more particularly to devices of this type which are intended to operate with a minimum of wear between the various moving parts.
In the past a number of different roller-type and ball-type transmissions have been proposed and produced, and have met with varying degrees of commercial success.
One such arrangement is shown in U.S. Pat. No. 3,296,880 issued to Arthur M. Maroth, which discloses a screw and nut device wherein the nut body is provided with multiple rollers having special conical tips which were adapted to engage modified square threads of the screw. The rollers were carried on anti-friction bearings such that most of the friction between the rollers and the surface of the screw threads was of the rolling kind, as opposed to the sliding type. This particular device functioned quite well, although it tended to be expensive to manufacture and assemble due to the relatively large number of parts involved and the close tolerances associated with such parts, which were necessary in order to insure reliable and trouble-free performance.
Another type of mechanism of the kind described is disclosed in U.S. Pat. No. 4,031,765 issued to Joseph R. Metz and having common ownership with the present application. The system shown therein involves a diamond-threaded screw and nut arrangement wherein a series of balls carried by the nut is capable of shifting in axial directions with respect thereto, making it possible for the nut to automatically reverse its axial direction of travel with respect to the screw when it arrives at either of the opposite ends thereof.
The particular thread form employed in the above device involved shallow thread grooves of scalloped configuration, such grooves being essentially semi-circular in cross section. With the application of relatively heavy loads to the nut, there was a pronounced tendency for a load carrying ball to hug or to ride on the sides of the grooves, and thus impose severe load stresses thereon. At these points, the groove surfaces did not cradle effectively the spherical surface of the balls, since in actuality the groove reactive forces involve hemispherical surfaces of the balls, which are not adequate to cradle or nest the balls for resisting loading. In consequence, the side surfaces of the grooves suffered excessive wear, as did the sockets of the nut body. Also, areas of the grooves near the crest of the screw were particularly susceptible to wear, since the balls tended to ride up the sides of the grooves to an appreciable extent, under heavy loading.
Ideally it was desired that all of the balls which were involved in carrying the load share the same equally. With the designs of the prior art devices, this was not always the case. Since devices of these types are not only employed in ticket printer mechanisms but also in automatic machinery where long life expectancy and freedom from premature failure are required, it was important to minimize, from the standpoint of the design of such parts, arrangements which were susceptible to jamming, binding or excessive wear. While these prior devices performed a useful functional life and had wear resistance up to a point, the poor loading conditions and resulting excessive wear appreciably limited their utility. Some prior devices, notably involving tapered rollers, had resilient backing means constituted of multiple components including at least one bowed spring washer. The make-up of the resilient backing, with its multiple components, represented an appreciable added cost in material and labor, which was especially undesirable where a large number of tapered rollers was required in a single transmission unit.